1. Field
Embodiments described herein relate to a nonvolatile semiconductor memory device.
2. Description of the Related Art
There is a rapidly increasing demand for a NAND flash memory with an increase in a use of large-capacity data, such as images or moving pictures in, for example, mobile apparatuses. In particular, the use of a multi-value storage technique capable of storing information of two bits or more in one memory cell enables a large amount of information to be stored in a small chip area.
In the high-density flash memories in which the memory cells are formed by a microfabrication technique, a memory cell with little deterioration immediately after shipment and a memory cell which deteriorates due to the repetition of write/erase operations have different threshold voltage change characteristics during the write/erase operations. In addition, a change in the threshold voltage through time after the write operation depends on the deteriorating state of the memory cell.